Device to-device (D2D) operation method performed by terminal in wireless communication system, and terminal using said method

ABSTRACT

Provided are a D2D operation method performed by a terminal in a wireless communication system, and a terminal using said method. The method comprises the steps of: receiving a public land mobile network (PLMN) list and performing a D2D on the basis of the PLMN list, wherein the PLMN list comprises at least one PLMN.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2015/008265, filed on Aug. 6, 2015,which claims the benefit of U.S. Provisional Application No. 62/034,113filed on Aug. 6, 2014, the contents of which are all hereby incorporatedby reference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a device-to-device (D2D) operation method performed bya terminal in a wireless communication system, and a terminal using themethod.

Related Art

In an International Telecommunication Union Radio communication sector(ITU-R),

a standardization of International Mobile Telecommunication(IMT)-Advanced being a next mobile communication system after a thirdgeneration has been performed. The IMT-Advanced is aimed at supportingan Internet Protocol (IP) based multi-media service with a datatransmission rate of 1 Gbps in a stop and low speed moving state and adata transmission rate of 1 Gbps in a high speed moving state.

A 3rd Generation Partnership Project (3GPP) is preparing LTE-Advanced(LTE-A) being an improved one of Long Term Evolution (LTE) based on anOFDMA (Orthogonal Frequency Division Multiple Access)/SC-FDMA (SingleCarrier-Frequency Division Multiple Access) transmission scheme as asystem standard satisfying requirements of IMT-Advanced. The LTE-A isone important candidate for IMT-Advanced.

In recent years, there is growing interest in a Device-to-Device (D2D)technology performing direct communication between devices. Inparticular, the D2D is attracting attention as a communicationtechnology for a public safety network. A commercial communicationnetwork has been rapidly changed to the LTE but a current public safetynetwork is based on a 2G technology in a collision problem and a costside with an existing communication standard. Request for the technologyclearance and an improved service induces an effort to improve thepublic safety network.

The public safety network has high service requirements (reliability andsecurity) as compared with a commercial communication network. Inparticular, when coverage of cellular communication is insufficient oris not used, there is a need for direct signal transmission/receptionbetween devices, that is, a D2D operation.

The D2D operation may be signal transmission/reception between adjacentdevices to have various advantages. For example, a D2D terminal mayperform data communication with a high transmission rate and low delay.Further, the D2D operation may distribute traffic converged in a basestation. If the D2D terminal serves as a relay, the D2D terminal mayserve to extend coverage of a base station.

Meanwhile, a network may broadcast information relating to adevice-to-device (D2D) operation in a specific cell, for example,information to indicate resources for receiving a D2D signal. It isneeded to specify how to operate when a terminal receives suchinformation.

SUMMARY OF THE INVENTION

A technical aspect of the present invention is to provide adevice-to-device (D2D) operation method performed by a terminal in awireless communication system, and a terminal using the method.

In an aspect, a method for device-to-device (D2D) operation in awireless communication system is provided. The method is performed by auser equipment (UE) and comprises receiving a public land mobile network(PLMN) list and performing D2D based on the PLMN list, wherein the PLMNlist comprises at least one PLMN.

The PLMN list may comprise information indicating a D2D poperationallowed in the PLMN.

The D2D operation allowed in the PLMN may comprise an operation ofallowing only D2D reception, an operation of allowing only D2Dtransmission, and an operation of allowing both D2D transmission and D2Dreception.

The method may further comprise determining priority order offrequencies based on the PLMN list and performing cell reselection basedon a determination.

The PLMN list may further comprise information indicating whether the UEis allowed to assign a top frequency priority to a frequency in whichD2D is to be performed in cell reselection.

In the determining of the priority order of frequencies based on thePLMN list, the UE may apply a top priority to a frequency belonging tothe PLMN list.

In another aspect, a user equipment (UE) is provided. The UE comprises aradio frequency (RF) unit that transmits and receives radio signals anda processor coupled with the RF unit to operate, wherein the processorthat: receives a public land mobile network (PLMN) list and performs D2Dbased on the PLMN list, and the PLMN list comprises at least one PLMN.

The PLMN list may comprises information indicating a D2D poperationallowed in the PLMN.

The D2D operation allowed in the PLMN may comprise an operation ofallowing only D2D reception, an operation of allowing only D2Dtransmission, and an operation of allowing both D2D transmission and D2Dreception.

The processor may determine priority order of frequencies based on thePLMN list and performs cell reselection based on a determination.

The PLMN list may further comprise information indicating whether the UEis allowed to assign a top frequency priority to a frequency in whichD2D is to be performed in cell reselection.

In the determining of the priority order of frequencies based on thePLMN list, the UE may apply a top priority to a frequency belonging tothe PLMN list.

According to the present invention, there is provided a D2D operationmethod performed by a terminal in a wireless communication system, and aterminal using the method.

According to the present invention, a terminal may receive a public landmobile network (PLMN) list from a network. Here, the terminal mayperform different operations based on a PLMN to provide a variety ofservices.

According to the present invention, the terminal may perform ProSedirect discovery according to the received PLMN. Here, differentoperations may be allowed for ProSe direct discoveries of PLMNs,respectively. The terminal may provide a variety of services.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane.

FIG. 3 is a diagram showing a wireless protocol architecture for acontrol plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate.

FIG. 5 is a flowchart illustrating a procedure of establishing RRCconnection.

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocedure.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

FIG. 8 illustrates sub states where the terminal may have in an RRC_IDLEstate and a sub state transition process.

FIG. 9 illustrates a reference structure for a ProSe.

FIG. 10 illustrates arrangement examples of terminals performing ProSedirect communication and cell coverage.

FIG. 11 illustrates a user plane protocol stack for the ProSe directcommunication.

FIG. 12 illustrates a PC 5 interface for D2D discovery.

FIG. 13 illustrates an embodiment of a ProSe direct discovery procedure.

FIG. 14 illustrates another embodiment of a ProSe direct discoveryprocedure.

FIG. 15 illustrates a D2D operating method of a terminal according to anembodiment of the present invention.

FIG. 16 illustrates a D2D operating method of a terminal according toanother embodiment of the present invention.

FIG. 17 is a block diagram illustrating a terminal according to anembodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides acontrol plane and a user plane to a user equipment (UE) 10. The UE 10may be fixed or mobile, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a mobile terminal (MT), a wireless device, etc. The BS 20is generally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC) 30, more specifically, to a mobility management entity (MME)through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway(P-GW). The MME has access information of the UE or capabilityinformation of the UE, and such information is generally used formobility management of the UE. The S-GW is a gateway having an E-UTRANas an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. Among them, a physical (PHY) layer belonging to the first layerprovides an information transfer service by using a physical channel,and a radio resource control (RRC) layer belonging to the third layerserves to control a radio resource between the UE and the network. Forthis, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane. FIG. 3 is a diagram showing a wireless protocol architecture fora control plane. The user plane is a protocol stack for user datatransmission. The control plane is a protocol stack for control signaltransmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with aninformation transfer service through a physical channel. The PHY layeris connected to a medium access control (MAC) layer which is an upperlayer of the PHY layer through a transport channel. Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channel is classified according to how and with whatcharacteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of atransmitter and a receiver, through a physical channel. The physicalchannel may be modulated according to an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme, and use the time and frequency as radioresources.

The functions of the MAC layer include mapping between a logical channeland a transport channel and multiplexing and demultiplexing to atransport block that is provided through a physical channel on thetransport channel of a MAC Service Data Unit (SDU) that belongs to alogical channel. The MAC layer provides service to a Radio Link Control(RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation,and reassembly of an RLC SDU. In order to guarantee various types ofQuality of Service (QoS) required by a Radio Bearer (RB), the RLC layerprovides three types of operation mode: Transparent Mode (TM),Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provideserror correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer isrelated to the configuration, reconfiguration, and release of radiobearers, and is responsible for control of logical channels, transportchannels, and PHY channels. An RB means a logical route that is providedby the first layer (PHY layer) and the second layers (MAC layer, the RLClayer, and the PDCP layer) in order to transfer data between UE and anetwork.

The function of a Packet Data Convergence Protocol (PDCP) layer on theuser plane includes the transfer of user data and header compression andciphering. The function of the PDCP layer on the user plane furtherincludes the transfer and encryption/integrity protection of controlplane data.

What an RB is configured means a procedure of defining thecharacteristics of a wireless protocol layer and channels in order toprovide specific service and configuring each detailed parameter andoperating method. An RB can be divided into two types of a Signaling RB(SRB) and a Data RB (DRB). The SRB is used as a passage through which anRRC message is transmitted on the control plane, and the DRB is used asa passage through which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRClayer of an E-UTRAN, the UE is in the RRC connected state. If not, theUE is in the RRC idle state.

A downlink transport channel through which data is transmitted from anetwork to UE includes a broadcast channel (BCH) through which systeminformation is transmitted and a downlink shared channel (SCH) throughwhich user traffic or control messages are transmitted. Traffic or acontrol message for downlink multicast or broadcast service may betransmitted through the downlink SCH, or may be transmitted through anadditional downlink multicast channel (MCH). Meanwhile, an uplinktransport channel through which data is transmitted from UE to a networkincludes a random access channel (RACH) through which an initial controlmessage is transmitted and an uplink shared channel (SCH) through whichuser traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that aremapped to the transport channel include a broadcast control channel(BCCH), a paging control channel (PCCH), a common control channel(CCCH), a multicast control channel (MCCH), and a multicast trafficchannel (MTCH).

The physical channel includes several OFDM symbols in the time domainand several subcarriers in the frequency domain. One subframe includes aplurality of OFDM symbols in the time domain. An RB is a resourcesallocation unit, and includes a plurality of OFDM symbols and aplurality of subcarriers. Furthermore, each subframe may use specificsubcarriers of specific OFDM symbols (e.g., the first OFDM symbol) ofthe corresponding subframe for a physical downlink control channel(PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval(TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logicallyconnected to the RRC layer of the E-UTRAN. A case where the RRC layer ofUE is logically connected to the RRC layer of the E-UTRAN is referred toas an RRC connected state. A case where the RRC layer of UE is notlogically connected to the RRC layer of the E-UTRAN is referred to as anRRC idle state. The E-UTRAN may check the existence of corresponding UEin the RRC connected state in each cell because the UE has RRCconnection, so the UE may be effectively controlled. In contrast, theE-UTRAN is unable to check UE in the RRC idle state, and a Core Network(CN) manages UE in the RRC idle state in each tracking area, that is,the unit of an area greater than a cell. That is, the existence ornon-existence of UE in the RRC idle state is checked only for each largearea. Accordingly, the UE needs to shift to the RRC connected state inorder to be provided with common mobile communication service, such asvoice or data.

When a user first powers UE, the UE first searches for a proper cell andremains in the RRC idle state in the corresponding cell. The UE in theRRC idle state establishes RRC connection with an E-UTRAN through an RRCconnection procedure when it is necessary to set up the RRC connection,and shifts to the RRC connected state. A case where UE in the RRC idlestate needs to set up RRC connection includes several cases. Forexample, the cases may include a need to send uplink data for a reason,such as a call attempt by a user, and to send a response message as aresponse to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types ofstates: EPS Mobility Management-REGISTERED (EMM-REGISTERED) andEMM-DEREGISTERED are defined. The two states are applied to UE and theMME. UE is initially in the EMM-DEREGISTERED state. In order to access anetwork, the UE performs a procedure of registering it with thecorresponding network through an initial attach procedure. If the attachprocedure is successfully performed, the UE and the MME become theEMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, twotypes of states: an EPS Connection Management (ECM)-IDLE state and anECM-CONNECTED state are defined. The two states are applied to UE andthe MME. When the UE in the ECM-IDLE state establishes RRC connectionwith the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in theECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1connection with the E-UTRAN. When the UE is in the ECM-IDLE state, theE-UTRAN does not have information about the context of the UE.Accordingly, the UE in the ECM-IDLE state performs procedures related toUE-based mobility, such as cell selection or cell reselection, without aneed to receive a command from a network. In contrast, when the UE is inthe ECM-CONNECTED state, the mobility of the UE is managed in responseto a command from a network. If the location of the UE in the ECM-IDLEstate is different from a location known to the network, the UE informsthe network of its corresponding location through a tracking area updateprocedure.

System information is described below.

System information includes essential information that needs to be knownby UE in order for the UE to access a BS. Accordingly, the UE needs tohave received all pieces of system information before accessing the BS,and needs to always have the up-to-date system information. Furthermore,the BS periodically transmits the system information because the systeminformation is information that needs to be known by all UEs within onecell. The system information is divided into a Master Information Block(MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include a limited number of parameters that are mostessential and most frequently transmitted when other information isrequired to be obtained from a cell. UE first searches for an MIB afterdownlink synchronization. The MIB may include information, such as anSFN that supports downlink channel bandwidth, a PHICH configuration, andsynchronization and operates as a timing criterion and an eNB transmitantenna configuration. The MIB may be transmitted on a broadcast channel(BCH) through broadcasting.

SystemInformationBlockType1 (SIB1) of included SIBs is included in a“SystemInformationBlockType1” message and transmitted. The remainingSIBs other than the SIB1 is included in a system information message andtransmitted. To map the SIBs to the system information message may beflexibly configured by a scheduling information list parameter includedin the SIB1. In this case, each of the SIBs is included in a singlesystem information message, and only SIBs having the same schedulingrequirement value (e.g. cycle) may be mapped to the same systeminformation message. Furthermore, a SystemInformationBlockType2 (SIB2)is always mapped to a system information message corresponding to thefirst entry within the system information message list of a schedulinginformation list. A plurality of system information messages may betransmitted within the same cycle. The SIB1 and all the systeminformation messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in an E-UTRAN, the SIB1 may bededicated-signaled in the state in which it includes a parameterconfigured like an existing configured value. In this case, the SIB1 maybe included in an RRC connection reconfiguration message andtransmitted.

The SIB1 includes information related to UE cell access, and defines thescheduling of other SIBs. The SIB1 may include information related tothe PLMN identifiers of a network, tracking area code (TAC) and a cellID, a cell barring status indicative of whether a cell is a cell onwhich camp-on is possible, the lowest reception level required within acell which is used as cell reselection criterion, and the transmissiontime and cycle of other SIBs.

The SIB2 may include radio resource configuration information common toall pieces of UE. The SIB2 may include information related to an uplinkcarrier frequency and uplink channel bandwidth, an RACH configuration, apage configuration, an uplink power control configuration, a soundingreference signal configuration, a PUCCH configuration supportingACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and detectinga change of system information to a primary cell (PCell) only. In asecondary cell (SCell), when a corresponding SCell is added, an E-UTRANmay provide all of pieces of system information related to an RRCconnection state operation through dedicated signaling. When systeminformation related to a configured SCell is changed, an E-UTRAN mayrelease an SCell that is taken into consideration and subsequently addthe changed system information. This may be performed along with asingle RRC connection reconfiguration message. An E-UTRAN may configureparameter values different from a value broadcasted within an SCell thathas been taken into consideration through dedicated signaling.

UE needs to guarantee the validity of a specific type of systeminformation, and such system information is called required systeminformation. The required system information may be defined as follows.

-   -   If UE is an RRC idle state: The UE needs to be guaranteed so        that it has the valid versions of the MIB and the SIB1 in        addition to the SIB2 to SIB8. This may comply with the support        of a radio access technology (RAT) that is taken into        consideration.    -   If UE is an RRC connection state: The UE needs to be guaranteed        so that it has the valid versions of the MIB, the SIB1, and the        SIB2.

In general, the validity of system information may be guaranteed up to amaximum of 3 hours after the system information is obtained.

In general, service that is provided to UE by a network may beclassified into three types as follows. Furthermore, the UE differentlyrecognizes the type of cell depending on what service may be provided tothe UE. In the following description, a service type is first described,and the type of cell is described.

1) Limited service: this service provides emergency calls and anEarthquake and Tsunami Warning System (ETWS), and may be provided by anacceptable cell.

2) Suitable service: this service means public service for common uses,and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communicationnetwork operators. This cell may be used by only communication networkoperators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell maybe classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be providedwith limited service. This cell is a cell that has not been barred froma viewpoint of corresponding UE and that satisfies the cell selectioncriterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be providedwith suitable service. This cell satisfies the conditions of anacceptable cell and also satisfies additional conditions. The additionalconditions include that the suitable cell needs to belong to a PublicLand Mobile Network (PLMN) to which corresponding UE may access and thatthe suitable cell is a cell on which the execution of a tracking areaupdate procedure by the UE is not barred. If a corresponding cell is aCSG cell, the cell needs to be a cell to which UE may access as a memberof the CSG.

3) A barred cell: this cell is a cell that broadcasts informationindicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts informationindicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate. FIG. 4 illustrates a procedure in which UE that is initiallypowered on experiences a cell selection procedure, registers it with anetwork, and then performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) inwhich the UE communicates with a Public Land Mobile Network (PLMN), thatis, a network from which the UE is provided with service (S410).Information about the PLMN and the RAT may be selected by the user ofthe UE, and the information stored in a Universal Subscriber IdentityModule (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs tocells having measured BS and signal intensity or quality greater than aspecific value (cell selection) (S420). In this case, the UE that ispowered off performs cell selection, which may be called initial cellselection. A cell selection procedure is described later in detail.After the cell selection, the UE receives system informationperiodically by the BS. The specific value refers to a value that isdefined in a system in order for the quality of a physical signal indata transmission/reception to be guaranteed. Accordingly, the specificvalue may differ depending on applied RAT.

If network registration is necessary, the UE performs a networkregistration procedure (S430). The UE registers its information (e.g.,an IMSI) with the network in order to receive service (e.g., paging)from the network. The UE does not register it with a network whenever itselects a cell, but registers it with a network when information aboutthe network (e.g., a Tracking Area Identity (TAI)) included in systeminformation is different from information about the network that isknown to the UE.

The UE performs cell reselection based on a service environment providedby the cell or the environment of the UE (S440). If the value of theintensity or quality of a signal measured based on a BS from which theUE is provided with service is lower than that measured based on a BS ofa neighboring cell, the UE selects a cell that belongs to other cellsand that provides better signal characteristics than the cell of the BSthat is accessed by the UE. This procedure is called cell reselectiondifferently from the initial cell selection of the No. 2 procedure. Inthis case, temporal restriction conditions are placed in order for acell to be frequently reselected in response to a change of signalcharacteristic. A cell reselection procedure is described later indetail.

FIG. 5 is a flowchart illustrating a procedure of establishing RRCconnection.

UE sends an RRC connection request message that requests RRC connectionto a network (S510). The network sends an RRC connection establishmentmessage as a response to the RRC connection request (S520). Afterreceiving the RRC connection establishment message, the UE enters RRCconnected mode.

The UE sends an RRC connection establishment complete message used tocheck the successful completion of the RRC connection to the network(S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocedure. An RRC connection reconfiguration is used to modify RRCconnection. This is used to establish/modify/release RBs, performhandover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifyingRRC connection to UE (S610). As a response to the RRC connectionreconfiguration message, the UE sends an RRC connection reconfigurationcomplete message used to check the successful completion of the RRCconnection reconfiguration to the network (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile networkoperator. Each mobile network operator operates one or more PLMNs. EachPLMN may be identified by a Mobile Country Code (MCC) and a MobileNetwork Code (MNC). PLMN information of a cell is included in systeminformation and broadcasted.

In PLMN selection, cell selection, and cell reselection, various typesof PLMNs may be considered by the terminal.

Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC ofa terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing locationregistration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a generalservice is provided to the terminal through the HPLMN or the EHPLMN, theterminal is not in a roaming state. Meanwhile, when the service isprovided to the terminal through a PLMN except for the HPLMN/EHPLMN, theterminal is in the roaming state. In this case, the PLMN refers to aVisited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available PublicLand Mobile Networks (PLMNs) and selects a proper PLMN from which the UEis able to be provided with service. The PLMN is a network that isdeployed or operated by a mobile network operator. Each mobile networkoperator operates one or more PLMNs. Each PLMN may be identified byMobile Country Code (MCC) and Mobile Network Code (MNC). Informationabout the PLMN of a cell is included in system information andbroadcasted. The UE attempts to register it with the selected PLMN. Ifregistration is successful, the selected PLMN becomes a Registered PLMN(RPLMN). The network may signalize a PLMN list to the UE. In this case,PLMNs included in the PLMN list may be considered to be PLMNs, such asRPLMNs. The UE registered with the network needs to be able to be alwaysreachable by the network. If the UE is in the ECM-CONNECTED state(identically the RRC connection state), the network recognizes that theUE is being provided with service. If the UE is in the ECM-IDLE state(identically the RRC idle state), however, the situation of the UE isnot valid in an eNB, but is stored in the MME. In such a case, only theMME is informed of the location of the UE in the ECM-IDLE state throughthe granularity of the list of Tracking Areas (TAs). A single TA isidentified by a Tracking Area Identity (TAI) formed of the identifier ofa PLMN to which the TA belongs and Tracking Area Code (TAC) thatuniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by theselected PLMN and that has signal quality and characteristics on whichthe UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting acell by a terminal.

When power is turned-on or the terminal is located in a cell, theterminal performs procedures for receiving a service byselecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a servicethrough the cell by always selecting a suitable quality cell. Forexample, a terminal where power is turned-on just before should selectthe suitable quality cell to be registered in a network. If the terminalin an RRC connection state enters in an RRC idle state, the terminalshould selects a cell for stay in the RRC idle state. In this way, aprocedure of selecting a cell satisfying a certain condition by theterminal in order to be in a service idle state such as the RRC idlestate refers to cell selection. Since the cell selection is performed ina state that a cell in the RRC idle state is not currently determined,it is important to select the cell as rapid as possible. Accordingly, ifthe cell provides a wireless signal quality of a predetermined level orgreater, although the cell does not provide the best wireless signalquality, the cell may be selected during a cell selection procedure ofthe terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTEis described with reference to 3GPP TS 36.304 V8.5.0 (2009-03) “UserEquipment (UE) procedures in idle mode (Release 8)”.

A cell selection procedure is basically divided into two types.

The first is an initial cell selection procedure. In this procedure, UEdoes not have preliminary information about a wireless channel.Accordingly, the UE searches for all wireless channels in order to findout a proper cell. The UE searches for the strongest cell in eachchannel. Thereafter, if the UE has only to search for a suitable cellthat satisfies a cell selection criterion, the UE selects thecorresponding cell.

Next, the UE may select the cell using stored information or usinginformation broadcasted by the cell. Accordingly, cell selection may befast compared to an initial cell selection procedure. If the UE has onlyto search for a cell that satisfies the cell selection criterion, the UEselects the corresponding cell. If a suitable cell that satisfies thecell selection criterion is not retrieved though such a procedure, theUE performs an initial cell selection procedure.

A cell selection criterion may be defined as in Equation 1 below.Following Equation 1 can be referred to as measurement for determiningwhether or not S-criterion is satisfied.Srxlev>0 AND Squal>0,  [Equation 1]where:Srxlev=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(rxlevminoffset))−P_(compensation),Squal=Q _(qualmeas)−(Q _(qualmin) +Q _(qualminoffset))

In this case, in Equation 1, the variables may be defined as in Table 1below.

TABLE 1 Srxlev Cell selection RX level value (dB) Squal Cell selectionquality value (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP)Q_(qualmeas) Measured cell quality value (RSRQ) Q_(rxlevmin) Minimumrequired RX level in the cell (dBm) Q_(qualmin) Minimum required qualitylevel in the cell (dB) Q_(rxlevminoffset) Offset to the signalledQ_(rxlevmin) taken into account in the Srxlev evaluation as a result ofa periodic search for a higher priority PLMN while camped normally in aVPLMN Q_(qualminoffset) Offset to the signalled Q_(qualmin) taken intoaccount in the Squal evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in a VPLMN Pcompensationmax(P_(EMAX) − P_(PowerClass), 0) (dB) P_(EMAX) Maximum TX power levelan UE may use when transmitting on the uplink in the cell (dBm) definedas P_(EMAX) in [TS 36.101] P_(PowerClass) Maximum RF output power of theUE (dBm) according to the UE power class as defined in [TS 36.101]

Qrxlevminoffset and Qqualminoffset, that is, signaled values, are theresults of periodic discovery for a PLMN having higher priority while UEcamps on a normal cell within a VPLMN, and may be applied only when cellselection is evaluated. As described above, during the periodicdiscovery of a PLMN having higher priority, UE may perform cellselection evaluation using parameter values stored from another cell ofthe PLMN having such higher priority.

After UE selects any cell through a cell selection procedure, theintensity or quality of a signal between the UE and a BS may be changeddue to the mobility of the UE or a change of a radio environment.Accordingly, if the quality of the selected cell is changed, the UE mayselect another cell providing better quality.

After the UE selects a specific cell through the cell selectionprocedure, the intensity or quality of a signal between the UE and a BSmay be changed due to a change in the mobility or wireless environmentof the UE. Accordingly, if the quality of the selected cell isdeteriorated, the UE may select another cell that provides betterquality. If a cell is reselected as described above, the UE selects acell that provides better signal quality than the currently selectedcell. Such a procedure is called cell reselection. In general, a basicobject of the cell reselection procedure is to select a cell thatprovides UE with the best quality from a viewpoint of the quality of aradio signal.

In addition to the viewpoint of the quality of a radio signal, a networkmay determine priority corresponding to each frequency, and may informthe UE of the determined priorities. The UE that has received thepriorities preferentially takes into consideration the priorities in acell reselection procedure compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cellaccording to the signal characteristics of a wireless environment. Inselecting a cell for reselection when a cell is reselected, thefollowing cell reselection methods may be present according to the RATand frequency characteristics of the cell.

-   -   Intra-frequency cell reselection: UE reselects a cell having the        same center frequency as that of RAT, such as a cell on which        the UE camps on.    -   Inter-frequency cell reselection: UE reselects a cell having a        different center frequency from that of RAT, such as a cell on        which the UE camps on    -   Inter-RAT cell reselection: UE reselects a cell that uses RAT        different from RAT on which the UE camps

The principle of a cell reselection procedure is as follows.

First, UE measures the quality of a serving cell and neighbor cells forcell reselection.

Second, cell reselection is performed based on a cell reselectioncriterion. The cell reselection criterion has the followingcharacteristics in relation to the measurements of a serving cell andneighbor cells.

Intra-frequency cell reselection is basically based on ranking. Rankingis a task for defining a criterion value for evaluating cell reselectionand numbering cells using criterion values according to the size of thecriterion values. A cell having the best criterion is commonly calledthe best-ranked cell. The cell criterion value is based on the value ofa corresponding cell measured by UE, and may be a value to which afrequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority providedby a network. UE attempts to camp on a frequency having the highestfrequency priority. A network may provide frequency priority that willbe applied by UEs within a cell in common through broadcastingsignaling, or may provide frequency-specific priority to each UE throughUE-dedicated signaling. A cell reselection priority provided throughbroadcast signaling may refer to a common priority. A cell reselectionpriority for each terminal set by a network may refer to a dedicatedpriority. If receiving the dedicated priority, the terminal may receivea valid time associated with the dedicated priority together. Ifreceiving the dedicated priority, the terminal starts a validity timerset as the received valid time together therewith. While the valid timeris operated, the terminal applies the dedicated priority in the RRC idlemode. If the valid timer is expired, the terminal discards the dedicatedpriority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE witha parameter (e.g., a frequency-specific offset) used in cell reselectionfor each frequency. For the intra-frequency cell reselection or theinter-frequency cell reselection, a network may provide UE with aNeighboring Cell List (NCL) used in cell reselection. The NCL includes acell-specific parameter (e.g., a cell-specific offset) used in cellreselection. For the intra-frequency or inter-frequency cellreselection, a network may provide UE with a cell reselection black listused in cell reselection.

The UE does not perform cell reselection on a cell included in the blacklist.

Ranking performed in a cell reselection evaluation procedure isdescribed below.

A ranking criterion used to give the priority of a cell is defined as inEquation 2.R _(s) =Q _(meas,s) +Q _(hyst) , R _(n) =Q _(meas,n) −Q_(offset)  [Equation 2]

In Equation 2, Rs is the ranking criterion of a serving cell on which UEnow camps, Rn is the ranking criterion of a neighboring cell, Qmeas,s isthe quality value of the serving cell measured by the UE, Qmeas,n is thequality value of the neighboring cell measured by the UE, Qhyst is ahysteresis value for ranking, and Qoffset is an offset between the twocells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between aserving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does notQoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for acorresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does notreceive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterionRn of a neighbor cell are changed in a similar state, ranking priorityis frequency changed as a result of the change, and UE may alternatelyreselect the twos. Qhyst is a parameter that gives hysteresis to cellreselection so that UE is prevented from to alternately reselecting twocells.

UE measures RS of a serving cell and Rn of a neighbor cell according tothe above equation, considers a cell having the greatest rankingcriterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality ofa cell is the most important criterion in cell reselection. If areselected cell is not a suitable cell, UE excludes a correspondingfrequency or a corresponding cell from the subject of cell reselection.

Hereinafter, radio link failure (RLF) will be described.

UE continues to perform measurements in order to maintain the quality ofa radio link with a serving cell from which the UE receives service. TheUE determines whether or not communication is impossible in a currentsituation due to the deterioration of the quality of the radio link withthe serving cell. If communication is almost impossible because thequality of the serving cell is too low, the UE determines the currentsituation to be an RLF.

If the RLF is determined, the UE abandons maintaining communication withthe current serving cell, selects a new cell through cell selection (orcell reselection) procedure, and attempts RRC connectionre-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken ascases where normal communication is impossible.

-   -   A case where UE determines that there is a serious problem in        the quality of a downlink communication link (a case where the        quality of a PCell is determined to be low while performing RLM)        based on the radio quality measured results of the PHY layer of        the UE    -   A case where uplink transmission is problematic because a random        access procedure continues to fail in the MAC sublayer.    -   A case where uplink transmission is problematic because uplink        data transmission continues to fail in the RLC sublayer.    -   A case where handover is determined to have failed.    -   A case where a message received by UE does not pass through an        integrity check.

An RRC connection re-establishment procedure is described in more detailbelow.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

Referring to FIG. 7, UE stops using all the radio bearers that have beenconfigured other than a Signaling Radio Bearer (SRB) #0, and initializesa variety of kinds of sublayers of an Access Stratum (AS) (S710).Furthermore, the UE configures each sublayer and the PHY layer as adefault configuration. In this procedure, the UE maintains the RRCconnection state.

The UE performs a cell selection procedure for performing an RRCconnection reconfiguration procedure (S720). The cell selectionprocedure of the RRC connection re-establishment procedure may beperformed in the same manner as the cell selection procedure that isperformed by the UE in the RRC idle state, although the UE maintains theRRC connection state.

After performing the cell selection procedure, the UE determines whetheror not a corresponding cell is a suitable cell by checking the systeminformation of the corresponding cell (S730). If the selected cell isdetermined to be a suitable E-UTRAN cell, the UE sends an RRC connectionre-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RATdifferent from that of the E-UTRAN through the cell selection procedurefor performing the RRC connection re-establishment procedure, the UEstops the RRC connection re-establishment procedure and enters the RRCidle state (S750).

The UE may be implemented to finish checking whether the selected cellis a suitable cell through the cell selection procedure and thereception of the system information of the selected cell. To this end,the UE may drive a timer when the RRC connection re-establishmentprocedure is started. The timer may be stopped if it is determined thatthe UE has selected a suitable cell. If the timer expires, the UE mayconsider that the RRC connection re-establishment procedure has failed,and may enter the RRC idle state. Such a timer is hereinafter called anRLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as anRLF timer. The UE may obtain the set value of the timer from the systeminformation of the serving cell.

If an RRC connection re-establishment request message is received fromthe UE and the request is accepted, a cell sends an RRC connectionre-establishment message to the UE.

The UE that has received the RRC connection re-establishment messagefrom the cell reconfigures a PDCP sublayer and an RLC sublayer with anSRB1. Furthermore, the UE calculates various key values related tosecurity setting, and reconfigures a PDCP sublayer responsible forsecurity as the newly calculated security key values. Accordingly, theSRB 1 between the UE and the cell is open, and the UE and the cell mayexchange RRC control messages. The UE completes the restart of the SRB1,and sends an RRC connection re-establishment complete message indicativeof that the RRC connection re-establishment procedure has been completedto the cell (S760).

In contrast, if the RRC connection re-establishment request message isreceived from the UE and the request is not accepted, the cell sends anRRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfullyperformed, the cell and the UE perform an RRC connection reconfigurationprocedure. Accordingly, the UE recovers the state prior to the executionof the RRC connection re-establishment procedure, and the continuity ofservice is guaranteed to the upmost.

FIG. 8 illustrates sub states where the terminal may have in an RRC_IDLEstate and a sub state transition process.

Referring to FIG. 8, a terminal performs an initial cell selectionprocess (S801). The initial cell selection process may be performed whenthere is no stored cell information with respect to the PLMN or asuitable cell is not found.

If the suitable cell is not found in the initial cell selection process,the terminal transitions to an any cell selection state (S802). Theoptional cell selection state represents a state which does not camp onin both of a suitable cell and an acceptable cell. The optional cellselection state is a state attempted by the terminal in order to find anacceptable cell of an optional PLMN which may camp on. When the terminalfinds no cells which may camp on, the terminal is continuouslymaintained in an optional cell selection state until the acceptable cellis found.

If the suitable cell is found in the initial cell selection process, thestate transits to a normal camp state (S803). The normal camp staterepresents a state which camps on the normal cell. A paging channel isselected according to information given through system information tomotor, and an evaluation process for cell reselection may be performed.

In the normal camp state (S803), if a cell reselection evaluationprocess (S804) is caused, the cell reselection evaluation process (S804)is performed. If a suitable cell is found in the cell reselectionevaluation process (S804), the terminal again transits to the normalcamp state (S803).

If an acceptable cell is found in the any cell selection state (S802),the terminal transits to an any cell camped state (S805). The any cellcamped state (S805) represents a state of camping on an acceptable cell.

In the any cell camped state (S805), the terminal may select a pagingchannel according to information given through system information tomonitor, and may perform a cell reselection evaluation process (S806).If the acceptable cell is not found in the cell reselection evaluationprocess (S806), the terminal transits the any cell selection state(S802).

Hereinafter, a D2D operation will be described. In the 3GPP LTE-A, aservice related to the D2D operation refers to Proximity based Services(ProSe). Hereinafter, the ProSe is an equivalent concept with the D2Doperation and the ProSe may be compatibly used with the D2D operation.The ProSe is now described.

The ProSe includes ProSe direct communication and ProSe directdiscovery. The ProSe direct communication presents communicationperformed by two or more adjacent terminals. The terminals may performcommunication using a protocol of a user plane. A ProSe-enabled UE meansa UE for supporting a process related to requirements of the ProSe.Unless otherwise defined, the ProSe-enabled UE includes both of a publicsafety UE and a non-public safety UE. The public safety UE represents aUE for supporting both of a public safety specified function and theProSe process. The non-public safety UE is a terminal which supports theProSe process but does not support the public safety specified function.

The ProSe direct discovery is a process where the ProSe-enabled UEdiscovers another ProSe-enabled UE. In this case, only ability of thetwo ProSe-enabled UEs is used. An EPC-level ProSe discovery signifies aprocess where an EPC determines whether 2 ProSe enable terminals areclosed to each other, and reports the close state thereof the two ProSeenabled terminals.

Hereinafter, the ProSe direct communication may refer to D2Dcommunication, and the ProSe direct discovery may refer to D2Ddiscovery.

FIG. 9 illustrates a reference structure for a ProSe.

Referring to FIG. 9, the reference structure for a ProSe includes aplurality of terminals having E-UTRAN, EPC, and ProSe applicationprogram, a ProSe application (APP) server, and a ProSe function.

An EPC is a representative example of the E-UTRAN. The EPC may includean MME, an S-GW, a P-GW, a policy and charging rules function (PCRF),and a home subscriber server (HSS).

The ProSe application server is a user of ProSe in order to make anapplication function. The ProSe application server may communicate withan application program in the terminal. The application program in theterminal may use a ProSe ability to make an application function.

The ProSe function may include at least one of following functions butis not limited thereto.

-   -   Interworking via a reference point towards the 3rd party        applications    -   Authorization and configuration of the UE for discovery and        direct communication)    -   Enable the function of the EPC level ProSe discovery    -   ProSe related new subscriber data and handling of data storage,        and also handling of ProSe identities    -   Security related function    -   Provide control towards the EPC for policy related function    -   Provide function for charging (via or outside of EPC, e.g.,        offline charging))

Hereinafter, a reference point and a reference interface will bedescribed in a reference structure for the ProSe.

-   -   PC1: a reference point between a ProSe application program in        the terminal and a ProSe application program in a ProSe        application server. The PC1 is used to define signaling        requirements in an application level.    -   PC2: is a reference point between the ProSe application server        and a ProSe function. The PC2 is used to define an interaction        between the ProSe application server and a ProSe function. An        application data update of a ProSe database of the ProSe        function may be an example of the interaction.    -   PC3: is a reference point between the terminal and the ProSe        function. The PC3 is used to define an interaction between the        terminal and the ProSe function. Configuration for ProSe        discovery and communication may be an example of the        interaction.    -   PC4: is a reference point between an EPC and the ProSe function.        The PC4 is used to define an interaction between the EPC and the        ProSe function. The interaction lay illustrate when a path for        1:1 communication or a ProSe service for real time session        management or mobility management are authorized.    -   PC5: is a reference point to use control/user plane for        discovery, communication, and relay between terminals, and 1:1        communication.    -   PC6: is a reference point to use a function such as ProSe        discovery between users included in different PLMNs.    -   SGi: may be used for application data and application level        control information exchange.

<ProSe Direct Communication (D2D Communication)>.

The ProSe direct communication is a communication mode where two publicsafety terminals may perform direct communication through a PC 5interface. The communication mode may be supported in both of a case ofreceiving a service in coverage of E-UTRAN or a case of separating thecoverage of E-UTRAN.

FIG. 10 illustrates arrangement examples of terminals performing ProSedirect communication and cell coverage.

Referring to FIG. 10(a), UEs A and B may be located outside of the cellcoverage. Referring to FIG. 10(b), the UE A may be located in the cellcoverage and the UE B may be located outside of the cell coverage.Referring to FIG. 10(c), both of UEs A and B may be located in the cellcoverage. Referring to FIG. 10(d), the UE A may be located in coverageof a first cell and the UE B may be in coverage of a second cell.

As described above, the ProSe direct communication may be performedbetween terminals which are provided at various positions.

Meanwhile, following IDs may be used in the ProSe direct communication.

Source layer-2 ID: The source layer-2 ID identifies a sender of a packetin a PC 5 interface.

Purpose layer-2 ID: The purpose layer-2 ID identifies a target of apacket in a PC 5 interface.

SA L1 ID: The SA L1 ID represents an in an ID in a scheduling assignment(SA) in the PC 5 interface.

FIG. 11 illustrates a user plane protocol stack for the ProSe directcommunication.

Referring to FIG. 11, the PC 5 interface includes a PDCH layer, a RLClayer, a MAC layer, and a PHY layer.

There may not be HARQ feedback in the ProSe direct communication. An MACheader may include the source layer-2 ID and the purpose layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>.

A ProSe enable terminal may use following two modes with respect toresource assignments for the ProSe direct communication.

1. Mode 1

The mode 2 is a mode for receiving scheduling a resource for the ProSedirect communication from a base station. The terminal should be in aRRC_CONNECTED state according to the mode 1 in order to transmit data.The terminal requests a transmission resource to the base station, andthe base station schedules a resource for scheduling assignment and datatransmission. The terminal may transmit a scheduling request to the basestation and may transmit a Buffer Status Report (ProSe BSR). The basestation has data which the terminal will perform the ProSe directcommunication and determines whether a resource for transmitting thedata is required.

2. Mode 2

The mode 2 is a mode for selecting a direct resource. The terminaldirectly selects a resource for the ProSe direct communication from aresource pool. The resource pool may be configured by a network or maybe previously determined.

Meanwhile, when the terminal includes a serving cell, that is, when theterminal is in an RRC_CONNECTED state with the base station or islocated in a specific cell in an RRC_IDLE state, the terminal isregarded to be in coverage of the base station.

If the terminal is located outside of the coverage, only the mode 2 isapplicable. If the terminal is located in the coverage, the mode 1 orthe mode 2 may be used according to setting of the base station.

If there are no exceptional conditions, only when the base station isconfigured, the terminal may change a mode from the mode 1 to the mode 2or from the mode 2 to the mode 1.

<ProSe Direct Discovery (D2D Discovery)>

The ProSe direct discovery represents a process used to discover whenthe ProSe enabled terminal discovers other neighboring ProSe enabledterminal and refers to D2D direction discovery or D2D discovery. In thiscase, an E-UTRA wireless signal through the PC 4 interface may be used.Hereinafter, information used for the ProSe direct discovery refers todiscovery information.

FIG. 12 illustrates a PC 5 interface for D2D discovery.

Referring to FIG. 12, the PC 5 interface includes an MAC layer, a PHYlayer, and a ProSe Protocol layer being an upper layer. Permission forannouncement and monitoring of discovery information is handled in theupper layer ProSe Protocol. Contents of discovery information aretransparent to an access stratum (AS). The ProSe Protocol allows onlyvalid discovery information to be transferred to the AS forannouncement.

An MAC layer receives discovery information from the upper layer ProSeProtocol. An IP layer is not used for transmitting the discoveryinformation. The MAC layer determines a resource used in order toannounce the discovery information received from the upper layer. TheMAC layer makes and sends a protocol data unit (MAC PDU) to a physicallayer. An MAC header is not added.

There are two types of resource assignments for announcing the discoveryinformation.

1. Type 1

The type 1 is a method assigned so that resources for announcing thediscovery information are not terminal-specific and the base stationprovides resource pool configuration for announcing the discoveryinformation to the terminals. The configuration may be included in asystem information block (SIB) to be signaled in a broadcast scheme.Alternatively, the configuration may be included in a terminal specificRRC message to be provided. Alternatively, the configuration may bebroadcast-signaled or terminal-specific signaled of a different layerfrom the RRC message.

The terminal selects a resource from an indicated resource pool toannounce discovery information using the selected resource. The terminalmay announce discovery information through a resource optionallyselected during each discovery period.

2. Type 2

The type 2 is a method where resources for announcing the discoveryinformation are terminal-specifically assigned. A terminal in aRRC_CONNECTED state may request a resource for announcing a discoverysignal to the base station through a RRC signal. The base station mayassign a resource for announcing a discovery signal as an RRC signal. Aresource for monitoring the discovery signal in a configured resourcepool may be assigned in terminals.

With respect to a terminal in an RRC_IDLE state, a base station mayreport a type 1 resource pool for announcing the discovery signal as anSIB. Terminals where ProSe direct discovery is allowed use a type 1resource pool for announcing the discovery information in the RRC_IDLEstate. Alternatively, the base station 2) reports that the base stationsupports the ProSe direct discovery through the SIB but may not providethe resource for announcing the discovery information. In this case, theterminal should enter the RRC_CONNECTED state for announcing thediscovery information.

With respect to a terminal in an RRC_CONNECTED state, the base stationmay configure whether to use a type 1 resource pool or a type 2 resourcepool for announcing the discovery information through a RRC signal.

FIG. 13 illustrates an embodiment of a ProSe direct discovery procedure.

Referring to FIG. 13, it is assumed in a terminal A and a terminal Bthat a ProSe-enabled application program is operated, and the terminal Aand the terminal B are configured in a friend relationship to eachother, that is, a relationship capable of allowing D2D communicationwith each other in the application program. Hereinafter, the terminal Bmay be expressed as a friend of the terminal A. For example, theapplication program may be a social networking program. 3GPP Layerscorrespond to functions of an application program for using a ProSediscovery service regulated according to 3GPP.

A ProSe direct discovery between the terminal A and the terminal B mayperform a following procedure.

1. First, the terminal A performs regular application-Layercommunication with an application server. The above communication isperformed based on Application programming interface (API).

2. A ProSe enabled application program of the terminal A receives a listof application layer IDs having a friend relationship. The applicationlayer ID may generally be in the form of a network access ID. Forexample, an application layer ID of the terminal A may have a form suchas adam@example.com.

3. A terminal A requests private expression codes for a user and privateexpression codes for a friend of the user.

4. 3GPP layers transmit an expression code request to a ProSe server.

5. The ProSe server map application layer IDs provided from an operatoror a third application server to private expression codes. For example,an application layer ID such as adam@example.com. The mapping may beperformed based on parameters (e.g., mapping algorithms, key values, andthe like) received from an application service of the network.

6. The ProSe server responds the obtained expression codes to the 3GPPlayers. The 3GPP layers report that expression codes with respect to therequested application layer are successively received to the ProSeenabled application program. Further, a mapping table between theapplication layer IDs and the expression codes are generated.

7. The ProSe enabled application program requests the 3GPP layers tostart the discovery procedure. That is, when one of friends is locatedclose to the terminal A and direct communication may be performed, theProSe enabled application program attempts the discovery. 3GPP layersannounce a private expression code of the terminal A (that is,“GTER543$#2FSJ67DFSF” which is a private expression code ofadam@example.com in the above example). In mapping of an applicationlayer ID of a corresponding application program and the privateexpression code, the mapping relationship may be known by the previouslyreceived friends, and the mapping may be performed.

8. It is assumed that the terminal B is operating the same ProSe enabledapplication program as that of the terminal A, and the above steps 3 to6 may be executed. 3GPP layers included in the terminal B may performProSe discovery.

9. When the terminal B receives the above announce from the terminal A,the terminal B determines whether the private expression code includedin the announce is known by the terminal B or is mapped to anapplication layer ID. As illustrated in step 8, since the terminal Bperforms steps 3 to 6, the terminal B knows a private expression codewith respect to the terminal A, mapping of the private expression codeto the application layer ID, and which is a corresponding applicationprogram. Accordingly, the terminal B may discover the terminal B fromthe announce of the terminal A. The 3GPP layers in the terminal Bannounces that adam@example.com is discovered to the ProSe enableapplication program.

FIG. 13 illustrates a discovery procedure by taking into considerationthe terminals A and B, the ProSe server, and the application server.Only an operation side between the terminals A and B is described. Theterminal A transmits a signal called the announce (the procedure mayrefer to announcement), and the terminal B receives the announce todiscover the terminal A. That is, a discovery procedure of FIG. 13 in anoperation directly related to another terminal among operationsperformed by each terminal may refer to a single step discoveryprocedure may refer to a single step discovery procedure in a side ofone step.

FIG. 14 illustrates another embodiment of a ProSe direct discoveryprocedure.

In FIG. 14, it is assumed that the terminal 1 to the terminal 4 may beincluded in a specific group communication system enablers (GCSE) group.It is assumed that the terminal 1 is a discoverer and terminals 2, 3,and 4 are a discoveree. A terminal 5 is a terminal regardless of adiscovery procedure.

The terminal 1 and the terminals 2 to 4 may perform a followingoperation in a discovery procedure.

First, the terminal 1 broadcasts a targeted discovery request message(hereinafter referred to ‘discovery request message’ or ‘M1’) in orderto discover whether an optional terminal included in the GCSE group islocated around the terminal 1. The targeted discovery request messagemay include a unique application program group ID or a layer-2 group IDof the specific GCSE group. Further, the targeted discovery requestmessage may include a unique ID of the terminal 1, that is, anapplication program private ID. The targeted discovery request messagemay be received by the terminals.

The terminal 5 transmits no response messages. The terminals 2, 3, and 4included in the GCSE group transmit a targeted discovery responsemessage (hereinafter referred to as a discovery response message or M2)as a response to the targeted discovery request message. The targeteddiscovery response message may include a unique application programprivate ID of a terminal transmitting the message.

An operation of terminals in a ProSe discovery procedure illustrated inFIG. 14 will be described. A discoverer (UE 1) transmits the targeteddiscovery request message, and receives a targeted discovery responsemessage being a response thereto. In addition, if a discoveree (e.g., UE2) receives the targeted discovery request message, the discovereetransmits a targeted discovery response message as a response thereto.Accordingly, each terminal performs an operation a second step. In theabove side, a ProSe discovery procedure of FIG. 14 may refer to adiscovery procedure.

In addition to the discovery procedure illustrated in FIG. 14, if theterminal 1 (discoverer) transmits a discovery confirm message(hereinafter may refer to M3) as a response to the targeted discoveryresponse message, this may refer to a third step discovery procedure.

Hereinafter, the present invention is described.

As described above, a network may provide PLMN information for aterminal. The PLMN information transmitted by the network to theterminal may be, for example, a PLMN list, which may indicate a singlePLMN or a plurality of PLMNs. Terminal-specific signaling or broadcastsignaling may be used to provide the PLMN information to the terminal.

The terminal may recognize based on the PLMN information received fromthe network that a different D2D operation is allowed for each PLMN. Theterminal may select a different cell for performing a D2D operationaccording to a D2D operation to perform with reference to the PLMNinformation.

For example, the terminal may be allowed to perform only the receptionof a D2D signal in a first PLMN, while the terminal may be allowed toperform both the transmission and the reception of a D2D signal in asecond PLMN. In this case, in selecting a cell for performing a D2Doperation, if the terminal intends to perform only D2D reception, mayselect any cell of a first PLMN cell and a second PLMN cell to perform aD2D operation. However, if the terminal intends to perform D2Dtransmission, the terminal should not select a second PLMN cell.

Suppose that the terminal selects a specific PLMN and the specific PLMNallows only the reception of a D2D signal. If the terminal intends totransmit a D2D signal, for example, to announce discovery information,no transmission of a D2D signal is allowed in the specific PLMN, andthus the selected PLMN is inappropriate.

FIG. 15 illustrates a D2D operating method of a terminal according to anembodiment of the present invention.

Referring to FIG. 15, a terminal receives PLMN information from anetwork (S1510). The PLMN information may be a PLMN list, which mayindicate at least one or more PLMNs.

The PLMN information may include information indicating a D2D operationallowed for the terminal in each PLMN. For example, suppose that thePLMN information includes a list including a first PLMN and a secondPLMN. The PLMN information may include information indicating that theterminal is allowed to perform both the transmission and the receptionof a D2D signal in the first PLMN and information indicating that theterminal is allowed to perform only the reception of a D2D signal in thesecond PLMN.

The PLMN information may also include information indicating that theterminal is not allowed to perform both the transmission and thereception of a D2D signal in a specific PLMN. That is, the PLMNinformation may include information indicating that no D2D operation issupported in the specific PLMN. Further, instead of transmittinginformation indicating that neither transmission nor reception of a D2Dsignal is possible in the specific PLMN, the network may transmit PLMNexcluding the specific PLMN to the terminal. The transmission of a D2Dsignal may be the transmission of the foregoing announcement, and thereception of a D2D signal may be the foregoing monitoring.

Table 2 below illustrates an example of PLMN information includinginformation on a D2D operation supported in each PLMN.

TABLE 2 -- ASN1START SystemInformationBlockType19-r12 ::= SEQUENCE {discConfig-r12 SEQUENCE { discRxPool-r12 SL- DiscRxPoolList-r12,discTxPoolCommon-r12 SL- DiscTxPoolList-r12 OPTIONAL, -- Need ORdiscTxPowerInfo-r12 SL- DiscTxPowerInfoList-r12 OPTIONAL, -- Cond TxdiscSyncConfig-r12 SL- SyncConfigList-r12 OPTIONAL -- Need OR }OPTIONAL, -- Need OR discInterFreqList-r12 SL- CarrierFreqInfoList-r12OPTIONAL, -- Need OR lateNonCriticalExtension OCTET STRING OPTIONAL, ...} SL-CarrierFreqInfoList-r12 ::= SEQUENCE (SIZE (1..maxFreq)) OF SL-CarrierFreqInfo-r12 SL-CarrierFreqInfo-r12 ::= SEQUENCE {carrierFreq-r12 ARFCN-ValueEUTRA-r9, plmn-IdentityList-r12PLMN-IdentityList4-r12 OPTIONAL -- Need OP } PLMN-IdentityList4-r12 ::=SEQUENCE (SIZE (1..maxPLMN-r11)) OF PLMN- IdentityInfo2-r12PLMN-IdentityInfo2-r12 ::= CHOICE { plmn-Index-r12 INTEGER(1..maxPLMN-r11), plmnIdentity-r12 PLMN-Identity allowable operationENUMERATION {TX, RX, BOTH} } -- ASN1STOP

In Table 1, ‘plmn-IdentityList’ represents a list of PLMN IDs indicatedby ‘carrierFreq’ and may be an example of PLMN information.‘plmn-IdentityList’ may include a plurality of pieces of‘PLMN-IdentityInfo2’, and a specific PLMN may be indicated by‘plmn-Index’ and ‘plmnIdentity’ included in each piece of‘PLMN-IdentityInfo2’. ‘allowable operation’ is a field indicating a D2Doperation allowed or supported in the specific PLMN, wherein TXindicates that transmission is allowed, Rx indicates that reception isallowed, and BOTH indicates that both transmission and reception areallowed.

Hereinafter, for convenience of description, the terms ‘announcement’and ‘D2D transmission’ are replaced with each other, and the terms‘monitoring’ and ‘D2D’ are replaced with each other.

The terminal performs a D2D operation based on the PLMN information(S1520). As described above, the PLMN information may be a PLMN list,which may indicate at least one or more PLMNs. The PLMN information mayinclude information indicating a D2D operation allowed for the terminalin each PLMN.

When the terminal performs the D2D operation based on the PLMNinformation, the terminal may selectively the priority order offrequencies in cell selection and/or reselection. Hereinafter, a methodfor a terminal to selectively apply the priority order of frequencies incell selection and/or reselection when performing a D2D operation basedon PLMN information will be described. Specifically, a method for aterminal to selectively apply the priority order of frequencies in cellreselection will be illustrated to clearly describe the presentinvention.

FIG. 16 illustrates a D2D operating method of a terminal according toanother embodiment of the present invention.

Referring to FIG. 16, a terminal receives a PLMN list from a network(S1610). The PLMN list may include at least one or more PLMNs and mayinclude information indicating a frequency of each PLMN. Further, thePLMN list may include information indicating a D2D operation allowed forthe terminal in each PLMN.

For example, suppose that the PLMN list received by the terminal fromthe network includes a first PLMN, a second PLMN, a third PLMN, and afourth PLMN. Here, the PLMN list may include information indicating thatthe terminal is allowed to perform both the transmission and thereception of a D2D signal in the first PLMN, information indicating thatthe terminal is allowed to perform only the reception of a D2D signal inthe second PLMN, and information indicating that the terminal is allowedto perform only the transmission of a D2D signal in the third PLMN.Further, the PLMN list may include information indicating that theterminal is not allowed to perform both the transmission and thereception of a D2D signal in the fourth PLMN.

The PLMN list may further include information indicating whether theterminal is allowed to apply the priority order of frequencies in cellreselection, and the terminal may control the priority order offrequencies through the information.

Further, the PLMN list may further include information indicatingwhether the terminal is allowed to camp on a specific frequency byapplying the priority order of frequencies in cell reselection. Thespecific frequency may be a frequency which ProSe direct discovery isprovided for and the terminal is to perform ProSe direct discover in.

In addition, the PLMN list may further include information indicatingthat the announcement of ProSe direct discovery is allowed even in afrequency not belonging to the PLMN list.

Moreover, the PLMN list may also include information indicating that theannouncement of ProSe direct discovery is allowed when at least one ofPLMNs belonging to the PLMN list (for example, PLMN list#1) of theterminal is identical to a PLMN in another PLMN list (for example, PLMNlist#2) included in system information.

The terminal may determine the priority order of frequencies in cellreselection based on the PLMN list (S1620). More specifically, theterminal determines the priority order of frequencies in cellreselection based on the information indicating the D2D operationallowed for the terminal in each PLMN.

For example, suppose that the terminal wants to transmit and receive aD2D signal but the terminal is allowed only to receive a D2D signal in aPLMN currently set for the terminal. Here, the terminal may set apriority for a frequency allowed to both transmit and receive a D2Dsignal in order to perform cell reselection. Accordingly, even though asignal (or quality) of a cell of the frequency allowed to both transmitand receive a D2D signal is inferior to a signal (quality) of a cell ofa frequency allowed only to receive (transmit) a D2D signal, theterminal may reselect the cell of the frequency allowed to both transmitand receive a D2D signal.

Hereinafter, methods for a terminal to determine the priority order offrequencies based on a PLMN list are described in detail separately 1.in the case where the terminal applies a priority to a frequencyincluded in a PLMN list set for the terminal and 2. in the case wherethe terminal assigns a cell reselection priority for a frequency notincluded in the PLMN list set for the terminal.

1. When Applying the Top Priority to the Frequency Belonging to the PLMNList

In ProSe direct discovery on an inter-frequency, the terminal may assigna top priority to a frequency belonging to the PLMN list set for theterminal, in which the frequency assigned the top priority correspondsto a frequency providing the ProSe direct discovery. While the terminalis camping on a cell belonging to the PLMN list, the terminal is allowedto perform both monitoring and announcement.

Here, the terminal may determine whether to apply the top frequencypriority for the cell in the PLMN list based on ‘information indicatingwhether the terminal is allowed to apply a top frequency priority’ ofsystem information received through a network. Further, the terminal maydetermine whether the terminal is allowed to camp on a specificfrequency by applying a top frequency priority when reselecting the cellin the PLMN list based on ‘information indicating whether the terminalis allowed to camp on a specific frequency by applying a top frequencypriority in cell reselection’ of the system information received throughthe network. Here, the specific frequency may be a frequency in whichProSe direct discovery is provided and the terminal is to perform ProSedirect discovery.

The PLMN list set for the terminal may include an RPLMN and an EPLMN,and may include only an RPLMN. That is, when a cell is not included inthe PLMN list, the terminal may not allow a top priority for a frequencynot included in the PLMN in cell reselection. Here, when a specificfrequency supports ProSe direct discovery but is not included in thePLMN, the terminal may not allow a priority for the specific frequency.Here, the terminal applies a first option of inter-frequency (or cell)direct discovery methods to be mentioned below, instead of applyingtemporal camping as a second option of the inter-frequency (or cell)direct discovery methods. The two options of the inter-frequency directdiscovery methods are described in detail below.

Hereinafter, inter-frequency discovery may mean D2D discovery performedthrough a second frequency other than a first frequency that is aserving frequency of the terminal, and ‘ProSe direct discovery’ may bereferred to as ‘D2D discovery’ for convenience of description.

<Inter-Frequency Direct Discovery Method>

-   -   First option: the first option is a method in which the terminal        camps on a frequency that the terminal intends to apply a        priority in cell reselection and keeps monitoring the frequency.

That is, when the terminal using the first frequency as the servingfrequency intends to perform a D2D discovery operation in the secondfrequency, the terminal performs cell reselection by assigning apriority (precedence) to the second frequency and camps on the secondfrequency to perform a D2D discovery operation according to the firstoption.

The foregoing first option may be applied to inter-PLMN (that is, thesame PLMN) inter-frequency (that is, different frequencies) discovery.However, the first option is difficult to apply when a PLMN including acell with a different frequency does not belong to a PLMN equivalent toan RPLMN of the terminal, that is, an EPLMN, since the terminal may notbe allowed to camp on a frequency that the terminal intends to apply apriority and to keep monitoring the frequency with respect to theinter-PLMN discovery (in which the terminal, which has been connected toa first PLMN, connects a second PLMN to perform a D2D discoveryoperation). However, the first option may be inadequate for beingapplied for a plurality of frequencies belonging to the same PLMN andproviding direct discovery.

-   -   Second option: the second option is a method in which when the        terminal using the first frequency as the serving frequency        intends to perform a D2D discovery operation in the second        frequency, the terminal performs a D2D discovery operation just        temporarily on the second frequency, keeping camping on a cell        of the first frequency, instead of camping on a cell of the        second frequency.

For example, the terminal temporarily transfers to the second frequencyin a D2D discovery operation-enabled subframe to perform a D2D discoveryoperation and then returns to the first frequency in a next subframe.

To sum up, according to the second option, the terminal only temporarilytransfers to an inter-frequency in a direct discovery-enable time(subframe). The terminal in the idle mode only needs to measure pagingmonitoring and some signals sent by a serving cell and thus has asufficient time (subframe) for inter-frequency direct discoverymonitoring. The terminal may use another time (subframe) for anotherpurpose (for example, announcement and/or monitoring).

That is, the second option does not need special cell reselectionhandling using the priority order of frequencies. Further, with a propernumber of monitoring inter-frequencies, the second option may functionsuccessfully. Unlike the first option, the second option is applicableto inter-PLMN direct discovery as well as inter-PLMN. Further, if theterminal indicates a network of interest with respect to a ProSediscovery operation, the network may direct the terminal to a frequencyof interest through an RRC connection release including a handover orredirection setup.

2. When Applying the Top Priority to the Frequency Not Belonging to thePLMN List

In inter-frequency ProSe direct discovery (D2D discovery), when aspecific frequency does not belong to PLMN information but is afrequency providing ProSe direct discovery, the terminal may also assigna top frequency priority to the frequency. Here, the terminal assignsthe top priority for the specific frequency merely to temporarily stayfor ProSe direct discovery and does not camp on the specific frequencyto perform general communication.

In principle, when the terminal stays on a frequency with a priority notincluded in the PLMN list, the terminal is allowed only to perform ProSedirect discovery monitoring. Here, when the terminal receives‘information indicating that ProSe direct discovery announcement isallowed even in a frequency not belonging to a PLMN list of theterminal’ from a network, the terminal may perform ProSe directdiscovery announcement even in the frequency not belonging to the PLMNlist.

Further, when the terminal receives ‘information indicating that ProSedirect discovery announcement is allowed when any one of PLMNs belongingto the PLMN list (for example, PLMN list#1) of the terminal is identicalto a PLMN in another PLMN list (for example, PLMN list#2) included inthe system information,’ the terminal may perform ProSe direct discoveryannouncement even in a frequency not belonging to the PLMN list.

The terminal performs cell reselection based on the priority order offrequencies (S1630). Here, a detailed description on the terminalreselecting a cell based on the priority order has been described above.

FIG. 17 is a block diagram illustrating a terminal according to anembodiment of the present invention.

Referring to FIG. 17, the terminal 1100 includes a processor 1110, amemory 1120, and a radio frequency (RF) unit 1130. The processor 1110implements the proposed functions, procedures, and/or methods. Forexample, the processor 1110 may receive PLMN information from a networkand may perform a D2D operation based on the PLMN information.

Further, the processor 1110 may receive a PLMN list from the network,may determine the priority order of frequencies in cell reselectionbased on the PLMN list, and may perform cell reselection based on thedetermined priority order of frequencies.

The RF unit 1130 is connected with the processor 1110 and transmitsand/or receives radio signals.

The processor may include an application-specific integrated circuit(ASIC), a separate chipset, a logic circuit, and/or a data processingunit. The memory may include a read-only memory (ROM), a random accessmemory (RAM), a flash memory, a memory card, a storage medium, and/orother equivalent storage devices. The RF unit may include a base-bandcircuit for processing a radio signal. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememory and may be performed by the processor. The memory may be locatedinside or outside the processor, and may be coupled to the processor byusing various well-known means.

What is claimed is:
 1. A method for performing device-to-device (D2D)operation in a wireless communication system, the method performed by auser equipment (UE) and comprising: receiving public land mobile network(PLMN) list information, wherein the PLMN list information includes atleast one PLMN and frequency information for each of the at least onePLMN; and performing the D2D operation based on the at least one PLMNand the frequency information, wherein the PLMN list information furtherincludes information related to an allowable operation for each of theat least one PLMN wherein the D2D operation includes a D2D reception ora D2D transmission, wherein the allowable operation is one of onlyallowing the D2D reception, only allowing the D2D transmission, andallowing both the D2D transmission and the D2D reception, and wherein,when the UE performs a cell reselection, the UE selects a cell at whichboth the D2D transmission and the D2D reception are allowed over anothercell at which only one of the D2D transmission or the D2D reception isallowed and having better cell quality than the cell at which both theD2D transmission and the D2D reception are allowed.
 2. The method ofclaim 1, wherein the PLMN list information further includes priorityinformation indicating whether a highest priority is allocated to the atleast one PLMN.
 3. The method of claim 2, wherein the UE performs thecell reselection based on the priority information, and wherein the UEperforms the D2D operation after performing the cell reselection.
 4. Auser equipment (UE) performing a device-to-device (D2D) operation, theUE comprising: a transceiver; and a processor, operatively coupled withthe transceiver, wherein the processor is configured to: control thetransceiver to receive public land mobile network (PLMN) listinformation, wherein the PLMN list information includes at least onePLMN and frequency information for each of the at least one PLMN; andperform the D2D operation based on the at least one PLMN and thefrequency information, wherein the PLMN list information furtherincludes information related to an allowable operation for each of theat least one PLMN wherein the D2D operation includes a D2D reception ora D2D transmission, wherein the allowable operation is one of onlyallowing the D2D reception, only allowing the D2D transmission, andallowing both the D2D transmission and the D2D reception, and wherein,when the UE performs a cell reselection, the UE selects a cell at whichboth the D2D transmission and the D2D reception are allowed over anothercell at which only one of the D2D transmission or the D2D reception isallowed and having better cell quality than the cell at which both theD2D transmission and the D2D reception are allowed.
 5. The UE of claim4, wherein the PLMN list information further includes priorityinformation indicating whether a highest priority is allocated to the atleast one PLMN.
 6. The UE of claim 5, wherein the UE performs the cellreselection based on the priority information, and wherein the UEperforms the D2D operation after performing the cell reselection.